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### **Overview**
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Absolute calibration enforces standardization since it enables
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independent developers and/or vendors to exchange their WR gear while
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achieving absolute sub ns timing. Absolute calibrated devices can be
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used as "golden standards" for the [relative calibration
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procedure](https://www.ohwr.org/project/white-rabbit/wikis/Calibration).
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Absolute calibration is achieved by measuring the time relationship (Δ
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<sub>TXcal</sub>, Δ <sub>RXcal</sub>) between the external electrical
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phase planes of the PPS signal and the interface to the
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Electrical-Optical / Optical-Electrical converter (EO/OE, usually the
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electrical SFP connector) as shown in Figure 1. Note that EO/OE
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converters have their own calibration parameters that define the
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relationship between their electrical and optical phase planes.
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Calibration of EO/OE converters is not covered on this wiki
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page.
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electrical SFP connector) as shown in Figure
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1.
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![](/uploads/5ce29cb41174ccb1272012afd9dbc6bc/link-model-abs-cal.png)
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*Figure 1: White Rabbit link delay model for absolute calibration**
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Absolute calibration enables independent developers and/or vendors to
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exchange their WR gear while achieving sub ns timing synchronization.
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Absolute calibrated devices can be used as "golden standards" for the
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[relative calibration
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procedure](https://www.ohwr.org/project/white-rabbit/wikis/Calibration).
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EO/OE converters have their own calibration parameters that define the
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relationship between their electrical and optical phase planes.
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For some WR applications the electrical system can be broken into
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different parts (for example a [DIO
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FMC](https://www.ohwr.org/project/fmc-dio-5chttla/wiki) to be plugged
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onto a [SPEC](https://www.ohwr.org/project/spec/wiki) board). When the
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electrical phase plane of the PPS signal is defined on such an
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exchangeable module and future exchange is foreseen then separate
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calibration of each of these parts is necessary.
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Both, calibration procedures for EO/OE converters and exchangeable
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modules, are outside the scope of this wiki page. However, it should be
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noted that EO/OE converters and exchangeable modules should store their
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calibration parameters in EPROM such that the system can automatically
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reach absolute calibration during link initialization.
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### **Principle**
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